X-ray and gamma ray sources are presently being used in a wide array of medical and industrial machinery, and the breadth of such use expands from year to year. Consumer tend to notice medical and dental X-ray machines, but in addition to these applications there are baggage screening machines, CAT scan machines, non-destructive industrial inspection machinery and ion implantation machines used in the manufacture of silicon wafer computer chips. All require that high voltage generated within the device be contained, and furthermore that radiation be contained and directed. In particular, the ion implantation machinery increased in the 1980's and 1990's with the silicon chip boom.
In the past, lead itself or lead-polymer composites were used to make electrical insulator items. But there are numerous problems with the use of lead. One problem with lead is that it is toxic and thus subject to increasingly stringent legal controls. Another issue is that lead may not have the mechanical or electrical properties desired for a given application. Lead has been used in various forms in wide range of applications: machined, as a solid casting, as a solid encased within a matrix such as a polymer matrix, or as a filler. As a filler, it may be lead particles, tribasic lead-sulfate or lead-oxide particles or particles of a specified shape or size, or as a mixture with other materials such as tin. Tungsten shielding, or polymer-tungsten shielding has also been used. Examples of all of these methods may be found in the prior art.
In general, polymer-metal composites are materials having a polymer matrix containing particles of a metal compound intermixed therein. The polymer may advantageously have plastic properties allowing for ease of manufacture, but a wide variety of polymers are known for use in such composites. In the prior art, lead has been a particularly favored material for its density and ease of working. Tungsten has been favored more recently, despite cost concerns. Three characteristics in particular which make such materials desirable are electrical non-conductivity, radiological shielding ability, and high density.
There is a growing list of applications for which polymer-metal composite materials are either required or advantageous. Ion implantation machine source insulators, X-ray tube insulation, radioisotope housings, other castings and housings could benefit from the properties of polymer-metal composite materials. In the case of typical high voltage insulators for ion implantation machinery, a thick walled generally round or cylindrical part is created out of lead or polymer-lead-oxide ranging from an inch to several feet or more in long dimension and weighing anywhere up to 500 pounds. Wall thickness may range from ½ inch to several inches. Such parts must resist high voltages, shield against x-ray or gamma ray emission and hold a high vacuum state when connected to the vacuum chamber. High voltage X-ray shielding for X-ray tube insulators is generally thinner (often 0.070 inch thickness), generally smaller, and of different shape, having an aperture for the X-ray beam, but once again must offer high voltage insulation and radiation protection. The lead in such devices obviously presents an environmental challenge to manufacture, use and disposal.
In the processing of lead precursor filled plastics known in the art, specialized facilities, handling procedures, training and safety equipment must be used to protect the employees from the lead precursor they handle. Lead-based dust is a particular concern, being airborne and inhalable. Such dust may be generated during mixing, molding, deflashing, machining and finishing of final products such as insulators or shields, to say nothing of earlier stages of mining, smelting and refining of lead and the final disposal of the used product at the end of its useful life. Even during the life span of the product, it is illegal to sand, machine, alter or use the product in any way that will generate dust. All such processes must be carried out at special lead handling sites, and all waste dust from any of these processes must be collected in accordance with OSHA regulations and transported to hazardous waste land fills in accordance with OSHA and DES guidelines.
Various radio-opaque agents are known which are used for diverse applications. Importantly, however, certain families of compounds are disfavored as having many of the same issues as lead and lead oxides. For example, the barium family of compounds are almost without exception subject to regulation due to their toxic nature. It is not previously known to use such barium family compounds in amounts greater than 10% by volume, since the structures in which they are emplaced are radio-opaque, not radiation barriers.
Internalized by law into the manufacturing process, such safety issues dramatically increase the cost of such products, which in turn increases other medical or industrial costs.
One recent invention to deal with this issue is TUNGSTEN-PRECURSOR COMPOSITE, for which application Ser. No. 10/095,350 filed Mar. 9, 2002 in the name of the same inventor, Stuart J. McCord was filed and has been allowed. This invention addresses material and cost concerns of tungsten shielding by proposing the use of tungsten precursor materials which testing reveals to have favorable properties. However, an entire range of desirable properties is not attainable with a single family of compounds, and so additional compounds may be desirable in order to expand the range of properties which may be attained in a lead-free shield device. Cost, of course, is one issue. Availability is another, as are actual material properties. During prosecution of that patent, U.S. Pat. No. 5,548,125 issued to Sandback (RADIATION PROTECTIVE GLOVE) and U.S. Pat. No. 4,957,943 issued to McAllister et al (PARTICLE-FILLED MICROPOROUS MATERIALS) were cited by the examiner prior to allowance. However, the glove patent, for example, teaches a flexible material most likely to be extruded.
Other prior art cited includes U.S. Pat. No. 3,473,028 issued to Curry for X-RAY TUBE HOUSEING CONSISTING OF A DIELECTRIC MATERIAL WITH AN ELECTRICALLY CONDUCTIVE LINER, issued Oct. 14, 1969. The device disclosed is neither annular nor composed of truncated cone shapes. Much more importantly, it teaches towards use of a specific dielectric material and thus teaches away from the material of the invention, and for that reason may not be combined with prior art showing the materials of the present invention.
U.S. Pat. No. 5,443,775 to Brannon on Aug. 22, 1995 for PROCESS FOR PREPARING PIGMENTED THERMOPLASTIC POLYMER COMPOSITIONS AND LOW SHRINKING THERMOSETTING RESIN MOLDING COMPOSITION is directed towards making of desirable colors and refractive properties in polymer products and is thus not relevant prior art for the present invention.
U.S. Pat. No. 4,938,233 issued to Orrison, Jr. for RADIATION SHIELD on Jul. 3, 1990 teaches a flexible radiation shield not manufacturable by casting and not having thick walls suitable for high voltage insulation. Since the device teaches flexibility, it teaches away from thick walls and thus cannot be combined with a device having useful high voltage insulation properties (i.e. having thick walls).
U.S. Pat. No. 7,079,624 to Miller et al for X-RAY TUBE AND METHOD OF MANUFACTURE, granted Jul. 18, 2006, teaches a device having an entirely different configuration, and teaches away from barium sulfate in a polymer matrix.
Another attempt to deal with the issue of environmental lead contamination may be found in U.S. Pat. No. 6,048,379 issued Apr. 11, 2000 to Bray et al for “HIGH DENSITY COMPOSITE MATERIAL”. This patent teaches the use of tungsten powder, a binder and a polymer to provide a composite material offering a density high enough for use as ammunition. As stated, a serious issue with the use of tungsten is that of cost. Tungsten metal is quite expensive in comparison to lead. For example, tungsten-composite materials may cost as much as 20$ per pound.
U.S. Pat. Nos. 5,730,664, 5,719,352, and 5,665,808, respectively issued to Asakura, Griffin, Bilsbury all disclose metal-polymer composites for projectiles, respectively golf balls and shot pellets. Other patents from the same art (projectiles) also propose non-toxic materials.
In the actual radiation shielding art itself, various patents propose polymer-metal composites of various forms.
EcoMASS (a registered trademark of the PolyOne Corporation) is a combination of tungsten metal and nylon and elastomer compounds used for shielding, apparently based upon the Bray '379 patent related to ammunition and thus developed specifically in response to military/sporting needs for non-toxic ammunition. It does not teach that materials other than tungsten may be used, thus limiting the range of characteristics of the final product. For example, tungsten is electrically conductive and thus is not normally suitable for insulators. As mentioned earlier, this material also faces cost limitations. In addition, this material has manufacturing limitations in terms of thickness and size of the final item.
U.S. Pat. No. 4,619,963 issued Oct. 28, 1986 to Shoji et al for “RADIATION SHIELDING COMPOSITE SHEET MATERIAL” teaches a lead-tin fiber and resin shield, as does U.S. Pat. No. 4,485,838 issued Dec. 4, 1984 to the same inventors. Obviously the lead inclusion leads to toxicity and thus regulation questions.
U.S. Pat. No. 6,310,355 issued Oct. 30, 2001 to Cadwalader for “LIGHTWEIGHT RADIATION SHIELD SYSTEM” teaches a flexible matrix having a radiation attenuating material and at least one void.
U.S. Pat. No. 6,166,390 issued Dec. 26, 2000 to Quapp et al for “RADIATION SHIELDING COMPOSITION” teaches a concrete composite material.
U.S. Pat. No. 5,360,666 issued Nov. 1, 1994 and U.S. Pat. No. 5,190,990 issued Mar. 2, 1993 to Eichmiller for “DEVICE AND METHOD FOR SHIELDING HEALTHY TISSUE DURING RADIATION THERAPY” teach a radiation shield for the human body comprising an elastomeric material and certain mixtures (see the summary of the invention) of various metals in the form of spherical particles.
Various metals might be explored for lead replacement. In such cases, it is natural enough to skip metals having families which are generally considered toxic or too expensive, and to skip those generally used in radio-opaque applications rather than radiological blocking applications. Thus, it would be natural to skip the barium family of compounds, since these are highly regulated.
It would be preferable to explore the use of other materials which are non-toxic and thus considerably safer than lead or certain available alternatives.